The present invention concerns dual axis dynamic focus of an electron beams used in a display monitor, for example, in a cathode ray tube display terminal.
Variations in the shape of display monitors are limited by what is acceptable to a user. Typically, users prefer the surface of a viewing screen of a display monitor to be as flat as possible. Further, it is typically desired that the distance from the viewing screen to the back of the display monitor be as small as possible, thus limiting the distance an electron beam generated by an electron gun can travel before colliding with the viewing screen.
The above-described geometric constraints in manufacture of a display monitor result in a large variation in distance the electron beam travels before colliding with the viewing screen. The distance the electron beam travels is dependent upon the radial distance from the center of the viewing screen to the location the electron beam collides with the viewing screen. Calculating the exact distance the electron beam travels would typically require the application of a fairly complex function dependent upon the actual shape and curvature of the viewing screen.
Because of the disparity in distance the electron beam travels before colliding with the viewing screen, it is typical to use a variable electrostatic lens to vary the focal distance of the electron beam in order to assure a sharp focus at the point of collision with the viewing screen. A focus voltage is applied to a grid on an electron gun in order to lengthen a focal distance of the electron beam from an initial focal distance. Typically the initial focal distance is the distance from the electron gun to the center of the viewing screen. The relationship of the focus voltage to the focal distance of the electron beam is approximately linear.
In the prior art various methods have been used to generate the focus voltage. One method includes the use of a diode array to implement a polynomial which approximates the voltage required to generate correct focal distances. Another method is to measure and store the voltage required to achieve the correct focus distance for each discrete location on the viewing screen. Thereafter the stored values are utilized in order to generate the correct focal distance for each discrete location on the viewing screen. These solutions, however, are complex and costly to implement.